He probably is the father, and it was a very stupid thing for him to do, especially in light of the current state of political affairs (no pun intended) and his wife's condition. I'm sure he's feeling pretty badly about it, and rightly so, as his political career in SC could be over.
Funny though how you can write it up like you did as though the very thought a democrat could do something like this just makes the entire party horrible, yet how many "moral" troubles have the republicans been having lately?
He made a very bad judgment call and will reap the rewards with his political career, or lack thereof. However, Edwards' moral failing may only result in the creation of a baby, a ruined marriage, and maybe the end of his career. How far reaching have some of the moral failings of republicans been of late?